Conventionally, there is proposed a technique in which, for performing exposure control in an image pickup device, a backlight level showing an intensity of backlight is detected based on a luminance level in a low luminance level region and a luminance level in another region in an image, and control parameters for gradation correction (gamma correction) are set to raise the luminance level in the low luminance level region in accordance with the backlight level. An example of such technique is disclosed in JP-A-2003-116049.
In the conventional technique, gradation is expanded in the low luminance level region so that contrast can be emphasized in a local region with a low luminance level.
The conventional technique, which for correcting gradation to raise the luminance level in a low luminance level region so as to emphasize contrast in a local region with a low luminance level in an image, can be applied not only to setting of control parameters in exposure control but also to image processing aimed at adjustment of the quality of a picked-up image which has been acquired by a device such as an image pickup device.
In the conventional technique however, since gradation correction is performed to raise the luminance level in a low luminance level region, the luminance level after the gradation correction becomes much higher than the luminance level before the gradation correction in the low luminance level region.
As a result of the gradation correction a difference in luminance level (contrast) between the low luminance level region and a high luminance level region is inevitably reduced in the image as a whole. That is, in the technique according to the background art, there is a problem that the global contrast of the image is lowered due to the gradation correction.